As electronic apparatuses have had a reduced size and included dense arrangement in their packages, circuit-forming boards having electronic components mounted thereon employ double-sided and multi-layer boards which can have more circuits and more components mounted thereon.
A conventional circuit forming board disclosed in Japanese Patent Laid-Open Publication No. 6-268345 will be explained. FIG. 6 illustrates a method of manufacturing prepreg sheet 13, a material of the board. A fiber sheet, such as glass cloth 11, used as a reinforcing member is dipped into varnish 12, impregnating impregnation material containing solvent having thermosetting resin diluted therein, thus impregnating glass cloth 11 with a predetermined amount of varnish 12. In order to adjust the amount of varnish 12, varnish 12 is squeezed by rollers, and then, is heated to change into a semi-cured state (stage B). The glass cloth in the stage B containing varnish is cut into pieces having a predetermined size, thus providing prepreg sheets 13 as material of circuit forming boards.
As shown in FIG. 6, prepreg sheet 13 is cut into a rectangle having longitudinal direction 202. The longitudinal direction 202 is oriented along a machine direction (MD), a direction in which glass cloth 11 is transferred. Film is stuck onto each surface of prepreg sheet 13 by a method shown in FIGS. 7 and 8.
FIG. 7 is a perspective view illustrating a process of sticking the film on prepreg sheet 13. FIG. 8 illustrates prepreg sheet 13 and film 14 seen along direction 204 in the process. Prepreg sheet 13 is introduced between films 14 along direction 201 (MD), i.e. the longitudinal direction 202, and then, films 14 are pressed onto prepreg sheet 13 by heated rollers 15, thereby heating and pressing prepreg sheet 13. Since the resin impregnated in prepreg sheet 13 is in the semi-cured state, the resin melts due to the heating, hence allowing film 14 to adhere temporarily onto prepreg sheet 13. Then, films 14 are cut into pieces having predetermined sizes along line 203, hence providing laminated prepreg sheet 16.
A method of manufacturing the circuit forming board will be described hereinafter. FIGS. 9A-9G show sectional views of the circuit forming board for illustrating the method of manufacturing the board. FIG. 9A shows laminated prepreg sheet 16 including prepreg sheet 13 and films 14 adhering onto both surfaces of sheet 13. As shown in FIG. 9B, prepreg sheet 16 is pierced by a laser beam, thereby forming a via-hole 17. The hole 17 is then filled with conductive paste 18 by printing, as shown in FIG. 9C. Conductive paste 18 is formed by mixing metallic particles, such as copper, with thermosetting resin, such as epoxy resin. Then, as shown in FIG. 9D, films 14 are peeled off. Since a small amount of resin component of films 14 melting on the surfaces of prepreg sheet 13 to allow films 14 to be temporarily stuck onto prepreg sheet 13, films 14 can be peeled off easily. As shown in FIG. 9D, paste 18 projects from sheet 13 by the thicknesses of films 14 after the peeling of the films. Then, as shown in FIG. 9E, copper foils 19 are placed on upper and lower surfaces of sheet 13, and then is hot-pressed by a hot presser, such as a vacuum hot presser. The resin component of sheet 13 melts and undergoes molding and curing steps, and then conductive paste 18 is compressed. As a result, copper foils 19 on both surfaces of sheet 13 are electrically connected to paste 18, as shown in FIG. 9F. Then, copper foils 19 are etched to have a predetermined pattern for forming circuit 20, as shown in FIG. 9G, thus providing a double-sided circuit forming board.
In the foregoing method, sheet 13 shown in FIG. 9D, upon having an uneven thickness, causes variation of a compressing rate of paste 18 at the hot-pressing, as shown in FIGS. 9E and 9F. The variation causes variation of a resistance of paste 18 which connects circuits 20 on both surfaces of the circuit forming board, thereby adversely affecting quality and reliability of the board.
That is, via-hole 17 formed in a thick portion of prepreg sheet 13, as shown in FIG. 9D, reduces a compression rate of paste 18, hence increasing a resistance of paste 18.
On the contrary, via-hole 17 formed in a thin portion of sheet 13 increases the compression rate of paste 18, hence reducing a resistance of paste 18. As such, the compression rate actually varies from a standard rate estimated based on an amount of the metallic particles mixed with the thermosetting resin, hence reducing the reliability of the coupling.
If the circuit forming board is required to have equal resistances, for instance, for handling high frequency signals, it is undesirable to allow electric resistance of paste 18 to vary according to a position of the board.
Even after films 14 are stuck onto prepreg sheet 13 having a large variation of the thickness, the variation of the thickness of sheet 13 still remains, hence allowing the variation of the thickness of the sheet 13 to remain even in laminated prepreg sheet 16. This problem also occurs in cases other than the case that glass cloth 11, a woven fabric, is employed as the material of prepreg sheet 13, i.e., the fiber sheet as the reinforcing member. This problem may occur in the case that unwoven fabric is employed. Woven fabric is impregnated with an amount of varnish 12 less than unwoven fabric. A larger amount of varnish 12 is accordingly attached at the surfaces of prepreg sheet 13, hence allowing the variation of the thickness of the woven fabric to affect electrical properties of conductive paste 18.
As thinner circuit forming boards have been recently demanded, glass cloth 11 becomes thin. This increases the amount of varnish 12 attached at the surface of prepreg sheet 13, hence making the above problem more serious.
In order to prevent the problem, it is essential that prepreg sheet 13 shown in FIG. 9A has an even thickness.
However, an accuracy limitation of manufacturing apparatuses limits flatness of the thickness of prepreg sheet 13 to a certain level when varnish 12 is squeezed with rollers, as shown in FIG. 6, and when films 14 are stuck onto both surfaces of sheet 13.